Resonant sensors have been used in a wide range of sensing applications, such as to measure load, pressure, torque and fluid flow characteristics. The key element of these sensors is the resonator, an oscillating structure, which is designed such that its resonance frequency is a function of the measurand.
The most common sensing mechanism is for the resonator to be stressed as a force sensor. The applied stress effectively increases the stiffness of the resonator structure, which results in an increase in the resonator's natural frequency. The resonator provides a virtual digital frequency output, which is less susceptible to electrical noise and independent of the level and degradation of transmitted signals, offering good long-term stability. The frequency output is compatible with digital interfacing, requiring no analogue-to-digital conversion and therefore maintaining inherent high accuracy and low cost.
Resonator sensors often have a relatively high mechanical quality factor (Q-factor), which leads to a high resolution of frequency and hence high sensitivity. A high Q-factor also implies low energy losses from the resonator and therefore low power requirements to maintain the resonance, and better noise rejection outside the resonance frequency bandwidth, which simplifies the operating electronics. Resonant sensors have been made in a wide range of types, sizes and materials as described, for example, by Barthod C, Teisseyre Y, Gehin C and Gautier G in “Resonant force sensor using PLL electronics”, Sensors and Actuators A 104 pages 143 to 150 (2003).
Current resonators used for measuring force, pressure and torque make use of resistance- strain gauges. This technology is around 40 years old and the performance of strain gauges is generally limited by fatigue and creep.
The manufacture of resonators using such technology is relatively expensive and labour intensive and therefore difficult to automate. Moreover, the technology has almost reached its limit in terms of performance levels.